The child’s blood type, if both parents are O Positive, is overwhelmingly likely to be O Positive. Blood type is determined by specific proteins, known as antigens, found on the surface of red blood cells. The complete blood type designation, such as O Positive, involves two separate genetic systems: the ABO group, which determines the letter (A, B, AB, or O), and the Rh factor, which determines the positive or negative sign.
How the ABO Blood Group System Works
The ABO blood group is controlled by a single gene with three possible variations, called alleles: A, B, and O. Every person inherits two of these alleles, one from each parent, which combine to determine the final blood type. The A and B alleles are dominant, meaning that if a person inherits an A allele and an O allele (AO), their blood type will still be A.
Conversely, the O allele is recessive, meaning it is only expressed if an individual inherits two copies of it, resulting in the OO genotype. Since both parents in this scenario have O blood type, their only possible genetic makeup for the ABO system is OO. They cannot possess any A or B alleles because those dominant alleles would have resulted in an A, B, or AB blood type.
Since both parents are OO, each can only pass on an O allele to their child. When two O alleles combine, the resulting genotype is guaranteed to be OO, meaning the child must have type O blood.
Understanding the Rh Factor
The positive or negative sign attached to a blood type refers to the Rh factor, which is a protein called the D antigen found on the surface of red blood cells. This factor is inherited separately from the ABO blood group, and its inheritance pattern is straightforward. The presence of the D antigen makes a person Rh-positive, while its absence makes them Rh-negative.
The gene for the Rh factor also follows a dominant and recessive pattern. The Rh-positive trait (represented by the dominant allele, D) is strong, and the Rh-negative trait (represented by the recessive allele, d) is weak. A person is Rh-positive if they have at least one dominant D allele (genotype DD or Dd), and they are Rh-negative only if they inherit two recessive d alleles (genotype dd).
Since both parents are O Positive, they must each carry at least one dominant D allele. However, an Rh-positive person can have a genotype of DD (homozygous dominant) or Dd (heterozygous), meaning they may also carry a recessive d allele. This distinction in their genetic makeup is what creates a slight variation in the child’s possible Rh factor.
The Definitive Outcome: O Positive Parents Produce O Positive Children
Determining the Rh factor involves considering the parents’ specific Rh genotypes, which are not always known from the blood type alone. Because both parents are Rh-positive, they could each be DD (homozygous dominant) or Dd (heterozygous).
If either parent is homozygous dominant (DD), they will only pass on a D allele, guaranteeing the child will be Rh-positive (O Positive). If both parents are heterozygous (Dd), they are both Rh-positive but carry the recessive Rh-negative trait. In this specific scenario, there is a 75% chance the child will be Rh-positive (DD or Dd) and a 25% chance they will inherit a recessive d allele from each parent, resulting in an Rh-negative child (dd).
The most probable outcome is O Positive. However, the small chance of an O Negative child exists only if both parents carry the recessive Rh-negative gene (d), even though they express the dominant positive trait.
Practical Implications of Knowing Your Child’s Blood Type
Knowing a child’s blood type can have significant practical importance, primarily in a medical context. The most immediate application is for blood transfusions, where receiving an incompatible blood type can cause a dangerous immune reaction. Type O Negative blood is highly valued in emergency settings because it is the “universal donor,” meaning it can be given to patients of any blood type.
The Rh factor is particularly important during pregnancy, especially if a mother is Rh-negative and the baby is Rh-positive. This situation, known as Rh incompatibility, can lead to the mother’s immune system creating antibodies that attack the baby’s red blood cells, causing a condition called hemolytic disease of the newborn. While a mother who is O Positive is not at risk for this specific Rh incompatibility, her Rh status is still routinely checked during the first prenatal visit to prevent any potential complications in future pregnancies.